The invention relates to interconnecting devices for instrumentation and control.
Control systems often include intelligent field devices having digital processors. Examples of such devices include valve controllers and transmitters, such as those associated with, for example, temperature sensors, pressure sensors, and flow meters. Another type of intelligent field device is a field mounted process controller that sends control signals to, for example, a valve controller, based on process information received from one or more transmitters.
The functionality of different field devices may be incorporated into a single module. For example, a process controller, a valve controller, and a temperature sensor/transmitter may be incorporated into a single module. However, even with such consolidation of function, control systems typically require some mechanism for transmitting signals between devices, and for transmitting signals from devices to a central control station.
Due to cost constraints, connections between devices, and between the devices and a control station, are often provided using a single pair of wires in many instances, this same pair of wires is used to provide power to the devices.
Many proprietary protocols have been developed for powering field devices and transmitting signals between the devices using a single pair of wires. Two such protocols are the HART protocol and the FOXCOMM protocol.
The HART protocol specifies provision of a 4-20 mA analog control signal and a 1200 baud bidirectional digital data link on a pair of wires connected to a device. The 4-20 mA signal is used both to power the device and to provide a control signal to or from the device. The HART protocol achieves simultaneous analog and digital transmission by using a frequency shift keying (FSK) technique to overlay a bidirectional digital signal on the analog control signal.
The FOXCOMM protocol also overlays a bidirectional digital data signal on a 4-20 mA analog control and power signal. Different versions of the protocol use either 600 baud or 4800 baud digital signals. Yet another version uses the analog signal only for power delivery and transmits all information using digital signals.
In one general aspect, the invention features providing a powered Ethernet connection between two devices connected by an Ethernet connection that provides data communication between the devices. Electrical power is applied on to the Ethernet connection at the first device. At the second device, the electrical power is extracted and used to power the second device.
Embodiments may include one or more of the following features. For example, the Ethernet connection may include two pairs of wires, with a first pair of wires being used to transmit data from the first device to the second device and a second pair of wires being used to transmit data from the second device to the first device. In this case, applying electrical power on to the Ethernet connection at the first device may include applying a DC voltage across the two pairs of wires by coupling a first potential to the first pair of wires and a second potential to the second pair of wires, with the DC voltage being defined as a difference between the two potentials. For example, when each pair of wires is connected to a corresponding transformer at the first device, and each transformer includes a center-tapped primary winding, coupling potentials to the pairs of wires may include applying the DC voltage between the center taps of the primary windings of the two transformers. As an alternative, a center-tapped inductor may be connected across each pair of wires at the first device, and potentials may be coupled to the pairs of wires by applying the DC voltage between the center taps of the inductors. Electrical power may be extracted at the second device through center-tapped windings of transformers of the second device, or through center-tapped inductors coupled across the pairs of wires at the second device.
The Ethernet connection also may be implemented using a single pair of wires to transmit data from the first device to the second device, to transmit data from the second device to the first device, and to provide power from the first device to the second device. In this case, applying electrical power on to the Ethernet connection at the first device may include applying a DC voltage across the pair of wires by coupling a first potential to the first wire through an inductor and coupling a second potential to the second wire through an inductor, with the DC voltage being defined as a difference between the two potentials.
Each device may be, for example, a process controller, a field device, or an Ethernet hub. A field device may be operable to sense a process condition and to transmit information about the sensed process condition using the Ethernet connection, or to control a process condition in response to a command received through the Ethernet connection.
In another general aspect, the invention features connecting a four-terminal Ethernet connection to a two-wire Ethernet connection. A device having a four-terminal Ethernet connection, the connection including a first pair of terminals for transmitting data away from the device and a second pair of terminals for transmitting data to the device is connected to a two-wire Ethernet connection for transmitting data to and from the device by a switched connection. The switched connection operating in a first mode in which the two-wire Ethernet connection is connected to the first pair of terminals and a second mode in which the two-wire Ethernet connection is connected to the second pair of terminals. The switched connection is initially set to operate in the first mode, and the two-wire Ethernet connection is monitored for data being transmitted to the device. The switched connection is set to operate in the second mode upon detection of data being transmitted to the device.
Power may be injected on to the two-wire Ethernet connection at the device for use in powering another device connected to the two-wire Ethernet connection. In addition, an impedance presented to the two-wire Ethernet connection may be changed from a first impedance when the switched connection is operating in the first mode to a second impedance when the switched connection is operating in the second mode.
In another general aspect, the invention features a field device having a powered Ethernet connection. The field device includes communications circuitry operable to communicate data over an Ethernet connection using an Ethernet protocol, power circuitry operable to extract operating power from the Ethernet connection, and process circuitry operable to sense a process condition and to transmit information about the sensed process condition using the Ethernet connection, or to control a process condition in response to a command received through the Ethernet connection.
Examples of such a field device include a device providing a temperature or pressure sensor, a flow meter, and a valve controller. The communications circuitry and the power circuitry may be operable to interface with an Ethernet connection including only a single pair of wires.